Changes to IPv4

After giving a brief overview the important features of IPv6, we'll go into the details of the basics of IPv6. A brief understanding of how IPv4 works is assumed, and the changes in IPv6 will be highlighted. Starting with IPv6 addresses and how they're split up, we'll go into the various types of addresses there are, what became of broadcasts; then discuss the IP layer, changes for name resolving, and what's new in DNS for IPv6.

Addressing

An IPv4 address is a 32-bit value that's usually written in "dotted quad" representation, where each "quad" represents a byte value between 0 and 255, for example:

127.0.0.1

This allows a theoretical number of 2^32 or ~4 billion hosts to be connected on the Internet. Due to grouping, not all addresses are available today.

IPv6 addresses use 128-bit technology, which results in 2128 theoretically addressable hosts. This allows a really big number of machines to be addressed, and it will fit all today's requirements plus PDAs, cell phones, and even IP phones in the near future without any sweat. When writing IPv6 addresses, they are usually divided into groups of 16 bits written as four hex digits, and the groups are separated by colons. An example is:

fe80::2a0:d2ff:fea5:e9f5

This shows a special thing -- a number of consecutive zeros can be abbreviated by a single "::" once in the v6 number. The above address is thus equivalent to fe80:0:00:000:2a0:d2ff:fea5:e9f5 -- leading zeros within groups can be omitted.

To make addresses manageable, they are split in two parts, which are the bits identifying the network a machine is on, and the bits that identify a machine on a network or subnetwork. The bits are known as netbits and hostbits, and in both IPv4 and v6, the netbits are the "left," or most significant bits of an IP number; and the host bits are the "right," or least significant bits:

In IPv4, the border is drawn with the aid of the netmask, which can be used to mask all net/host bits. Typical examples are 255.255.0.0 which uses 16-bit for addressing the network, and 16-bit for the machine, or 255.255.255.0 which takes another 8 bits to allow addressing 256 subnets on, for example, a class B net.

When addressing switched from classful addressing to CIDR routing, the borders between net and host bits stopped being 8-bit boundaries, and as a result the netmasks started looking ugly and became unmanageable. As a replacement, the number of network bits is used for a given address, to denote the border. Thus

10.0.0.0/24

is the same as a netmask of 255.255.255.0 (24 single bits). The same scheme is used in IPv6:

2001:638:a01:2::/64

tells us that the address used here has the first (left-most) 64 bits used as the network address, and the last (right-most) 64 bits are used to identify the machine on the network. The network bits are commonly referred to as the (network) "prefix", and the prefix here would be 64 bits.

Common addressing schemes found in IPv4 are the (old) class B and class C nets. With a class C network (/24), 24 bits are assigned by your provider, which leaves 8 bits to be assigned by you. If you want to add any subnetting to that, you end up with "uneven" netmasks that are a bit tricky to deal with. Class B networks (/16) are easier cases where only 16 bits are assigned by the provider, and systems that allow subnetting, or splitting of the right-most bits into two parts -- one to address the on-site subnet, and one to address the hosts on that subnet. Usually, this is done on byte (8-bit) boundaries. Using a netmask of 255.255.255.0 (or a /24 prefix) allows flexible management even of bigger networks. Of course there is the upper limit of 254 machines per subnet, and 256 subnets.

With 128 bits available for addressing in IPv6, the scheme commonly used is the same, only the fields are wider. Providers usually assign /48 networks, which leaves 16 bits for a subnetting and 64 host bits.

IPv6 addresses have a similar structure to class B addresses.

Now while the space for network and subnets is sufficient, using 64 bits for addressing hosts seems like a waste. It's unlikely that you will want to have several billion hosts on a single subnet, so what is the idea behind this?

The idea behind having fixed-width, 64-bit wide host identifiers is that they aren't assigned manually as in IPv4. Instead, v6 host addresses are recommended (not mandated!) to be built from so-called EUI64 addresses. EUI64 addresses are -- as the name says -- 64-bits wide, and derived from MAC addresses of the underlying network interface. For example, with Ethernet, the 6-byte (48-bit) MAC address is usually filled with the hex bits "fffe" in the middle -- the MAC address

01:23:45:67:89:ab

results in the EUI64 address

01:23:45:ff:fe:67:89:ab

which again gives the host bits for the IPv6 address.

::0123:45ff:fe67:89ab

These host bits can now be used to automatically assign IPv6 addresses to hosts, which supports autoconfiguration of v6 hosts -- all that's needed to get a complete v6 IP number is the first (net/subnet) bits. IPv6 also offers a solution to assign them automatically.

When on a network of machines speaking IP, there's usually one router which acts as the gateway to outside networks. In IPv6 land, this router will send "router advertisement" information which clients are expected to either receive during operation or solicit upon startup. The router advertisement information includes data on the router's address, and which address prefix it routes. With this information and the host-generated EUI64 address, a v6-host can calculate its IP number, and there is no need for manual address assignment. Of course, routers still need some configuration.

The advertisement information routers create is part of the Neighbor Discovery Protocol (NDP, see [RFC2461]), which is the successor to IPv4's ARP protocol. In contrast to ARP, NDP does not only do lookup of v6 addresses for MAC addresses (the neighbor solicitation/advertisement part), but also does a similar service for routers and the prefixes they serve, which is used for autoconfiguration of v6 hosts as described in the last paragraph.